This invention relates to load pull testing of microwave transistors (DUT). When the transistors are driven in their nonlinear operation regime their internal output impedance is very low. An impedance tuner (see ref. 3) used to match the transistor in a load pull setup (FIG. 1) must also conjugate-match such impedance, i.e. the reflection factor presented by the tuner to the DUT must have the same amplitude and the opposite phase. The device under test (DUT) (3), is inserted between two impedance tuners (2) and (4) and signal (1) is injected from a signal source (9), which acts also as a receiver, and terminates at the load (5). The injected and reflected power waves are detected using bidirectional couplers (11) and (10) and fed into the receiver (9). The system is controlled by a PC controller (6) using tuner control (8) and data control (7) cables.
Passive impedance tuners (2), (4), see ref. 3, can reach maximum reflection factors |Γtuner| in the order of 0.95, corresponding (in a 50Ω system) to impedances of 2.4Ω. The insertion loss, created by RF cables, test fixtures, couplers (10), (11) etc. between DUT (3) and tuners (2) and (4) reduces the available tuning range at the DUT reference plane and thus the capacity of the passive tuner to match the transistor. The only remedy to this limitation is using active systems (see ref. 1 and 7), i.e. test systems whereby a signal coherent with the signal injected into the input of the DUT, is injected into the input or output of the DUT and creates “virtual” impedance (load). This additional signal can be the only one injected, in which case we speak of purely “active” load pull, or it can be superimposed to signal reflected by a passive tuner, in which case we speak of “hybrid” load pull; obviously if only a passive (mechanical or electronic) tuner is present, i.e. a tuner not comprising an amplifier, we speak of “passive” load pull. In both active injection cases (pure active and hybrid) the objective is to reach and match the internal impedance of the transistor DUT); in general terms a standard requirement is a tuning range reaching a reflection factor |Γ|=1 at the DUT reference plane (corresponding to an internal DUT impedance of 0Ω); due to above mentioned insertion losses, however, it is necessary that, at the tuner reference plane, the generated reflection factor Γtuner shall be |Γtuner|>1.
Mechanical tuners are slow (see ref. 3). The objective of this invention is a high speed active tuner apparatus using a fast digital electronic tuner (see ref. 3 and 6), an amplifier and forward signal injection mechanism allowing |ΓDUT|≥1. It has to be clarified at this point that “electronic” does not mean “active”. Electronic tuners, as disclosed here, are passive, but not mechanical (FIGS. 4 and 10).
Passive automatic (remote controlled) tuners are either electromechanical (see ref. 3) or electronic (see ref. 4 and 6 and FIG. 4, 10). Electromechanical tuners cover high frequency bandwidth (are wideband), generate high reflection factor (F, Gamma), are linear, have high tuning resolution, but they are slow, because of mechanical movement. Electronic tuners use PIN diodes (see ref. 4, 5 and FIG. 10), and have smaller bandwidth, lower maximum reflection factor Gamma, lower linearity and resolution than mechanical tuners, but they are extremely fast (they switch states in milli-seconds versus seconds of mechanical tuners); so in essence we are talking about an increased speed ratio (or reduced tuning time) of 500 to 1000:1. For a number of applications electronic tuners, if used in active configuration, as in this invention, can yield maximum reflection factor |ΓDUT|≥1 and can exploit their high tuning speed. And as modern test technologies evolve testing automatically a large number chips on-wafer, speed is of essence and may overcome other, comparative, weaknesses of electronic tuners.